One of the conventional information recording medium capable of three-dimensionally recording information is shown in FIG. 8 (Yoshimasa Kawata et al.: Three-dimensional optical memory using an organic multilayered recording medium; Optics Japan 2000, Proceedings, p. 95-96, 7pB12 (2000)). In this information recording medium, on a glass substrate 104, recording layers 101a to 101d made of urethane-urea copolymer material that is a photon-mode recording material, and intermediate layers 102a to 102c made of a PVA (polyvinyl alcohol) film and a PMMA (polymethyl methacrylate) film are laminated alternately.
Laser light 108 is focused (i.e., irradiation with convergent light 107) on a desired recording layer of the recording layers 101a to 101d of this information recording medium by an objective lens 106, whereby information can be recorded. The laser light 108 used herein is pulse laser light having a pulse width of about 100 femtoseconds and an extremely high peak power. By focusing such pulse laser light on the recording layers 101a to 101d, information is recorded on the recording layers 101a to 101d by using the two-photon absorption that is one of the non-linear absorption phenomena. Specifically, in a portion where a power density of the convergent light 107 is high (in a light-focusing point) in a region that is irradiated with the convergent light 107 in the recording layers 101a to 101d, the two-photon absorption occurs. In this state a phenomenon that looks as if the portion were irradiated with light with a half wavelength of the wavelength of actually irradiated light occurs, and an information bit 105 is written. Furthermore, on the information bit 105, low power light is focused, and light reflected therefrom is detected by a photodetector (not shown) via the objective lens 106, whereby signals can be reproduced. In this information recording medium, since a plurality of recording layers are laminated in the direction of an optical axis of the objective lens (z direction), information can be recorded three-dimensionally and the recording capacity is increased.
However, there has been a problem in the above-mentioned conventional information recording medium that the recording sensitivity of the recording layer was not good. Accordingly, in the case of a recording method in which one information bit is formed by one pulse, it is necessary to use a femtosecond laser having an extremely large peak power (about 100 kW) as a light source, and thus the structure of the light source becomes complicated. Furthermore, there has been another problem in that in the case where a light source with a smaller peak power is used, pulse irradiation at the same place is required to be repeated several times (for example, from several tens to several thousands times) (i.e., since a photon-mode recording material is used for a recording layer, accumulated recording of refractive index change is possible), whereby the speed of writing information is reduced.